1. Field of the Invention
This invention relates to a two-cycle internal combustion engine and, more particularly, to a two-cycle engine having intake and exhaust valves operated in response to a crank angle and a means for forming a swirl rotating around a cylinder axis.
2. Description of the Related Art
Japanese Examined Patent Publication No. 60-5770 discloses an open-chamber type two-cycle engine having intake and exhaust valves. In this two-cycle engine, when a piston is at bottom dead center, both valves are opened. Fresh air is introduced into a combustion chamber via the intake valve and along the cylinder wall so as to form a vertical loop flow. The boundary between the fresh air and the exhaust gas moves from a position in the vicinity of the intake valve to the center of the cylinder, and then to a position in the vicinity of the exhaust valve, so that the fresh air is substituted for the exhaust gas over the whole region in the combustion chamber.
In this known two-cycle engine, there are no particular problems when it is running in a heavy load condition. However, in an idling or light load condition, the flow of fresh air is to small that a large amount of exhaust gas remains in the cylinder, which prevents the fresh air from concentrating around the cylinder head having an ignition spark plug. This is because, according to the vertical loop flow, the main part of the fresh air moves to the lower part of the cylinder. Therefore, initial burning by the spark plug is disturbed and/or the generation of a combustion flame core is prevented. Accordingly, due to the reduction of flame transmitting speed, a misfire or change of combustion may occur. In this known art, a piston having a recess on the top face thereof is also disclosed. However, this recess does not serve to form a swirl rotating around a cylinder axis.
U.S. Pat. No. 4,543,928 to von Seggern discloses a two-cycle engine in which an air swirl is formed about a cylinder axis and stratification is obtained between the mixture in the ignition region and the air in the piston region. However, this swirl is not an exhaust gas swirl and no stratification is obtained between the exhaust gas and the fresh air.